Analytical Engine

In 1837, Charles Babbage was inspired by Jacquard's loom to attempt to build the Analytical Engine.[2] The names of the components of the calculating device were borrowed from the textile industry. In the textile industry, yarn was brought from the store to be milled. The device would have had a "store"--memory to hold 1,000 numbers of 40 decimal digits each. Numbers from the "store" would then have then been transferred to the "mill" (analogous to the CPU of a modern machine), for processing. It was programmed using two sets of perforated cards--one to direct the operation and the other for the input variables.[2][3] However, after more than 17,000 pounds of the British government's money, the thousands of cogged wheels and gears never fully worked together.[4]

During a nine-month period in 1842-43, Ada Lovelace translated the memoir of Italian mathematician Luigi Menabrea. The memoir covered the Analytical Engine. The translation contained Note G which completely detailed a method for calculating Bernoulli numbers using the Analytical Engine. This note is recognized by some historians as the world's first written computer program.[5]

Early programmable computers

The Z3 computer, invented by Konrad Zuse (1941) in Germany, was a digital and programmable computer.[9] A digital computer uses electricity as the calculating component. The Z3 contained 2,400 relays to create the circuits. The circuits provided a binary, floating-point, nine-instruction computer. Programming the Z3 was through a specially designed keyboard and punched tape.

The Electronic Numerical Integrator And Computer (Fall 1945) was a Turing complete, general-purpose computer that used 17,468 vacuum tubes to create the circuits. At its core, it was a series of Pascalines wired together.[10] Its 40 units weighed 30 tons, occupied 1,800 square feet (167 m2), and consumed $650 per hour (in 1940s currency) in electricity when idle.[10] It had 20 base-10accumulators. Programming the ENIAC took up to two months.[10] Three function tables were on wheels and needed to be rolled to fixed function panels. Function tables were connected to function panels using heavy black cables. Each function table had 728 rotating knobs. Programming the ENIAC also involved setting some of the 3,000 switches. Debugging a program took a week.[10] The ENIAC featured parallel operations. Different sets of accumulators could simultaneously work on different algorithms. It used punched card machines for input and output, and it was controlled with a clock signal. It ran for eight years, calculating hydrogen bomb parameters, predicting weather patterns, and producing firing tables to aim artillery guns.

The Manchester Small-Scale Experimental Machine (June 1948) was a stored-program computer.[11] Programming transitioned away from moving cables and setting dials; instead, a computer program was stored in memory as numbers. Only three bits of memory were available to store each instruction, so it was limited to eight instructions. 32 switches were available for programming.

Later computers

Computers manufactured until the 1970s had front-panel switches for programming. The computer program was written on paper for reference. An instruction was represented by a configuration of on/off settings. After setting the configuration, an execute button was pressed. This process was then repeated. Computer programs also were manually input via paper tape or punched cards. After the medium was loaded, the starting address was set via switches and the execute button pressed.[12]

Computer programming

Computer programming is the process of writing or editing source code. Editing source code involves testing, analyzing, refining, and sometimes coordinating with other programmers on a jointly developed program. A person who practices this skill is referred to as a computer programmer, software developer, and sometimes coder.

Imperative languages

A declaration couples a variable name to a datatype - for example: var x: integer;

An expression yields a value - for example: 2 + 2 yields 4

A statement might assign an expression to a variable or use the value of a variable to alter the program's control flow - for example: x := 2 + 2; if x = 4 then do_something;

One criticism of imperative languages is the side effect of an assignment statement on a class of variables called non-local variables.[16]

Declarative languages

Declarative programming languages describe what computation should be performed and not how to compute it. Declarative programs omit the control flow and are considered sets of instructions. Two broad categories of declarative languages are functional languages and logical languages. The principle behind functional languages (like Haskell) is to not allow side effects, which makes it easier to reason about programs like mathematical functions.[16] The principle behind logical languages (like Prolog) is to define the problem to be solved - the goal - and leave the detailed solution to the Prolog system itself.[17] The goal is defined by providing a list of subgoals. Then each subgoal is defined by further providing a list of its subgoals, etc. If a path of subgoals fails to find a solution, then that subgoal is backtracked and another path is systematically attempted.

Compilers are used to translate source code from a programming language into either object code or machine code.[18] Object code needs further processing to become machine code, and machine code consists of the central processing unit's native instructions, ready for execution. Compiled computer programs are commonly referred to as executables, binary images, or simply as binaries - a reference to the binaryfile format used to store the executable code.

Interpreters are used to execute source code from a programming language line-by-line. The interpreter decodes each statement and performs its behavior. One advantage of interpreters is that they can easily be extended to an interactive session. The programmer is presented with a prompt, and individual lines of code are typed in and performed immediately.

The main disadvantage of interpreters is computer programs run slower than when compiled. Interpreting code is slower because the interpreter must decode each statement and then perform it. However, software development may be faster using an interpreter because testing is immediate when the compiling step is omitted. Another disadvantage of interpreters is an interpreter must be present on the executing computer. By contrast, compiled computer programs need no compiler present during execution.

Just in time compilers pre-compile computer programs just before execution. For example, the Java virtual machine Hotspot contains a Just In Time Compiler which selectively compiles Java bytecode into machine code - but only code which Hotspot predicts is likely to be used many times.

Either compiled or interpreted programs might be executed in a batch process without human interaction.

No properties of a programming language require it to be exclusively compiled or exclusively interpreted. The categorization usually reflects the most popular method of language execution. For example, Java is thought of as an interpreted language and C a compiled language, despite the existence of Java compilers and C interpreters.

Storage and execution

Typically, computer programs are stored in non-volatile memory until requested either directly or indirectly to be executed by the computer user. Upon such a request, the program is loaded into random-access memory, by a computer program called an operating system, where it can be accessed directly by the central processor. The central processor then executes ("runs") the program, instruction by instruction, until termination. A program in execution is called a process.[19] Termination is either by normal self-termination or by error - software or hardware error.

Simultaneous execution

Many operating systems support multitasking which enables many computer programs to appear to run simultaneously on one computer. Operating systems may run multiple programs through process scheduling - a software mechanism to switch the CPU among processes often so users can interact with each program while it runs.[20] Within hardware, modern day multiprocessor computers or computers with multicore processors may run multiple programs.[21]

Multiple lines of the same computer program may be simultaneously executed using threads. Multithreading processors are optimized to execute multiple threads efficiently.

Self-modifying programs

A computer program in execution is normally treated as being different from the data the program operates on. However, in some cases, this distinction is blurred when a computer program modifies itself. The modified computer program is subsequently executed as part of the same program. Self-modifying code is possible for programs written in machine code, assembly language, Lisp, C, COBOL, PL/1, and Prolog.

Utility programs

Operating system

An operating system is a computer program that acts as an intermediary between a user of a computer and the computer hardware. [22]

In the 1950s, the programmer, who was also the operator, would write a program and run it.[12] After the program finished executing, the output may have been printed, or it may have been punched onto paper tape or cards for later processing.[12] More often than not the program did not work. [23] The programmer then looked at the console lights and fiddled with the console switches. If less fortunate, a memory printout was made for further study.[23] In the 1960s, programmers reduced the amount of wasted time by automating the operator's job.[23] A program called an operating system was kept in the computer at all times.[23]

Originally, operating systems were programmed in assembly; however, modern operating systems are typically written in C.

Embedded programs

Independent of the host computer, a hardware device might have embedded firmware to control its operation. Firmware is used when the computer program is rarely or never expected to change, or when the program must not be lost when the power is off.[23]

Microcode programs

Microcode programs control some central processing units and some other hardware. This code moves data between the registers, buses, arithmetic logic units, and other functional units in the CPU. Unlike conventional programs, microcode is not usually written by, or even visible to, the end users of systems, and is usually provided by the manufacturer, and is considered internal to the device.

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